Urodilatin (URO - ANP95-126), is a newly described member of the family of natriuretic peptides. It has the same structure as ANP99-126 with four additional amino acids on the amino terminal end. Likely, URO is of renal origin and secreted into urine. URO is more natriuretic than ANF at low doses. We investigated 1) The renal effects of exogenous infusion of URO and ANP99-126 before and after the induction of congestive heart failure (CHF) by creation of an aortocaval fistula (ACF) in rats, 2) The effects of pretreatment with neutral endopeptidase inhibitor (NEP-I), SQ28,063 on the renal response to URO in these rats, and 3) The effect of NEP-I and ANP clearance receptor blockade by C-ANF4-23 on the metabolism of 125I- ANP99-126 and 125I-URO. In control rats URO (0.75-12 microg/kg/h) caused dose dependent increases in urine flow (V), and absolute (UNaV) and fractional sodium excretion (FENa%). Glomerular filtration rate (GFR) rose significantly despite markedly lower mean arterial pressure (MAP). Infusion of the same doses of URO to rats with CHF induced significant increases in V, GFR, UNaV and FENa%, however the absolute levels of diuresis and natriuresis were lower in rats with CHF. When URO was infused into rats with ACF pretreated with NEP-I (40 mg/kg) the absolute urine flow and sodium excretion were not different from those obtained in control rats. Infusion of equimolar doses of ANP 99-126 into control rats resulted in diuretic and natriuretic response, which were smaller than those obtained after infusion of URO. Similarly, rats with CHF showed an attenuated renal response to ANP99-126, compared to their remarkable renal response to equimolar doses of URO. These differences between the natriuretic potency of ANP and URO, may be attributed to our findings that while ANP is removed from the circulation by NEP and ANP clearance receptors, URO is removed mainly by the latter.